1. Field of the Invention
The present invention relates to heterojunction bipolar transistors and more particularly, to a heterojunction bipolar transistor which is based on a heterojunction of AlGaAs/GaAs interface.
2. Description of the Related Art
Heterojunction bipolar transistors (HBTs), which are excellent in high frequency characteristics and switching characteristics, have been considered to be promising prospects as microwave transistors and high speed logical circuit transistors.
HBTs using GaAs, in particular, have been developed recently as superhigh speed devices which have a prospective future.
A major feature of the HBTs is that, since an emitter/base junction is a heterojunction, the impurity concentration of a base region can be made very high and a base resistance can be made low while preventing high impurity concentration of an emitter.
There have been recently proposed various types of structures of HBTs on the basis of the mobility characteristic of electrons in the GaAs, which can remarkably reduce a transit time of the electrons.
However, it is impossible to fully control the AlGaAs even with resorting to the use of the molecular-beam epitaxy (MBE) technique and this method involves various problems especially when an Al mole fraction exceeds 0.3.
When the Al mole fraction exceeds 0.3, the mobility of minority carriers becomes small and also DX centers decrease free carrier concentration significantly, which become serious obstacles to lowering the emitter resistance of the HBT.
Over the years, many studies have been made for making the operational speed of the HBT faster based on the scaling of ECL gates.
For the purpose of increasing the cut-off frequency of a transistor to efficiently lower a delay time t.sub.pd of the ECL gate, a self-alignment technique for forming shallow emitter and base as well as the optimum design of the transistor and ECL circuit based on the scaling become vital.
The ECL circuit comprises two sorts of transistors, i.e., transistors constituting a current switch circuit and a transistor as an emitter follower circuit.
In such an ECL circuit, in particular, the magnitude of a load resistance is determined by the magnitude of an emitter resistance. Accordingly, it is desirable to minimize the emitter resistance.
In the prior art HBT, however, a band gap difference of a valence band in an AlGaAs/GaAs heterojunction necessary for forming an npn type HBT must be about 150-200 meV from the viewpoint of its current gain. To this end, an Al mole fraction x in Al.sub.x Ga.sub.1-x As as the material of an emitter layer is set to be usually about 0.3. That is, the emitter layer is made of usually Al.sub.0.3 Ga.sub.0.7 As and the HBT using such an emitter layer is small in the mobility of the minority carriers as mentioned earlier, which disadvantageously results in that the emitter resistance is relatively large and thus the switching speed of the HBT cannot be made high.
In this way, the emitter layer of the conventional HBT is made of Al.sub.x Ga.sub.1-x As having an Al mole fraction of about 0.3. For this reason, the prior art HBT has had such a problem that the carrier mobility is small and the carrier life time is short and thus the performance of the HBT is insufficient.